Question

$$ \mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3} \longrightarrow \mathrm{CH}_{3}-\mathrm{CHO} $$ ' \(\mathrm{X}\) ' can be : (A) \(\mathrm{O}_{3}, \mathrm{Zn}\) (B) \(\mathrm{KMnO}_{4} / \mathrm{OH}, \Delta\) (C) (i) \(\mathrm{HCO}_{3} \mathrm{H}\), (ii) \(\mathrm{HIO}_{4}\) (D) Both (A) and (C)

Short answer

The correct reagent sequence to transform the alkene CH₃-CH=CH-CH₃ into the aldehyde CH₃-CHO is option (C), which includes (i) HCO₃H and (ii) HIO₄.

Step-by-step solution

Identify the starting material and the desired product

The given reaction involves conversion of the alkene, CH₃-CH=CH-CH₃, to the aldehyde, CH₃-CHO.

Review each option and determine its transformation capability

(A) O₃, Zn: Ozonolysis of alkenes followed by reduction using zinc will transform alkenes to carbonyl compounds such as aldehydes and ketones. However, the interior carbon in the starting molecule will be transformed into a ketone while the terminal carbon (the methyl group) remains unchanged. As a result, the desired product will not be formed using this reagent. (B) KMnO₄/OH, Δ: Alkaline potassium permanganate (KMnO₄) is a strong oxidizing agent that can transform an alkene to a 1,2-diol (glycol) through syn hydroxylation. Heating can then lead to cleavage of the C-C bond, usually producing carboxylic acids or ketones, not the desired aldehyde. (C) (i) HCO₃H, (ii) HIO₄: The given reagent sequence starts with the addition of an electrophile (HCO₃H) to the alkene, followed by the cleavage of the C-C bond using periodic acid (HIO₄). This reagent sequence is a common way to convert alkenes to aldehydes, specifically transforming the terminal alkene into an aldehyde.

Choose the correct option(s)

After analyzing each option, we can conclude that only option (C) can successfully transform the given alkene, CH₃-CH=CH-CH₃, into the desired aldehyde, CH₃-CHO. Therefore, the correct answer is (C).

Key concepts

Alkene to Aldehyde Conversion

Understanding how to convert alkenes to aldehydes is crucial for students studying organic chemistry, especially when preparing for competitive exams like JEE Advanced. Aldehydes are characterized by a carbonyl group (a carbon double-bonded to an oxygen) attached to at least one hydrogen atom. The challenge is in selectively oxidizing the alkene just enough to form the aldehyde, without going further to an acid or alcohol.

• Common Methods: There are several ways to achieve this transformation:
• Hydroboration-oxidation - adds water across the double bond with anti-Markovnikov selectivity, followed by oxidation to an aldehyde.
• Ozonolysis followed by reductive workup - cleaves the double bond and forms two carbonyl compounds, which can be selectively reduced to aldehydes.
• Using specific reagents like HCO₃H followed by cleavage with HIO₄ - adds an electrophile across the alkene to form aldehyde directly.

In our exercise, the latter method is preferred as it selectively converts the terminal alkene to an aldehyde. The initial step involves the addition of a 'formyl' group (from the HCO₃H) to the alkene, which then progresses to an oxidative cleavage by HIO₄, resulting in the formation of the desired aldehyde.

Ozonolysis of Alkenes

Ozonolysis is a fundamental reaction in organic chemistry where an alkene is cleaved with ozone (O₃), leading to the formation of carbonyl compounds. The steps involved in this reaction are quite interesting for students to grasp:

• Step 1: Addition of Ozone - Ozone reacts with the alkene to form an unstable ozonide compound.
• Step 2: Cleavage - The ozonide cleaves to form carbonyl compounds, which usually are aldehydes or ketones based on the structure of the original alkene.
• Step 3: Workup - By choosing zinc (Zn) as a reductant in the workup step, it reduces any carboxylic acids that may have formed to aldehydes or ketones.

However, for the purpose of selectively obtaining aldehydes from alkenes, especially terminal alkenes, ozonolysis needs to be carefully controlled. A reductive workup using Zn is often chosen to avoid over-oxidation to carboxylic acids, aiming for aldehyde formation. It's important for students to recognize that ozonolysis can lead to a mixture of products, which makes the choice of subsequent workup conditions crucial for determining the final outcome.

Oxidative Cleavage of Alkenes

Oxidative cleavage is a powerful tool in an organic chemist's arsenal, which involves the breaking of a carbon-carbon double bond by oxidation to form two carbonyl compounds from a single alkene. The type of products formed from this reaction depends on the reagents used:

• Potassium Permanganate (KMnO₄): Known for its robust oxidative ability, KMnO₄ can be used under acidic, neutral, or basic conditions to yield various products. Under alkaline conditions and elevated temperatures, it cleaves alkenes to form carboxylic acids or ketones.
• Ozonolysis: This implies a reaction with ozone, as described previously, and it is versatile in terms of the products it can generate. The nature of workup post-ozonolysis dictates the product's oxidation state.
• Reagents like HIO₄: When used in conjunction with electrophilic additions to the alkene, it can lead to oxidative cleavage, selectively generating aldehydes from terminal alkenes.

In the context of the exercise, using (i) HCO₃H and (ii) HIO₄ targets a cleaner conversion of the alkene to an aldehyde, thanks to the specificity of these reagents for the oxidative cleavage at the terminal position of an alkene. It’s essential for students to understand that the reagents and conditions chosen will define the outcome of the oxidative cleavage.